Superabrasive grit comprised of diamond and cubic boron nitride ("CBN") are widely used in sawing, drilling, dressing, and grinding applications. The grit is typically held in a matrix of nickel, copper, iron, cobalt, or tin, or alloys thereof, by mechanical bonds and the matrix is connected to a tool body. The matrix can also comprise a resin, such as phenol formaldehyde.
In mechanical bonding, the matrix surrounds the grit, holding them in place. While simple and practical, mechanical bonds are relatively weak and the grit can be easily lost as the surrounding matrix is abraded away during use. Grit retention can be improved by limiting the exposure of the grit by the matrix, but this decreases cutting efficiency. In a typical saw blade application, the average exposure of diamond grit is less than 20% of the total grit height. Grit loss can become a serious problem when the supporting matrix is worn down such that over one-third of the grit is exposed. The average lifetime of such cutting tools is decreased as up to two-thirds of the original diamond grit are prematurely lost.
In an attempt to improve grit retention, diamond particles have been coated with carbide forming transition metals. For example, U.S. Pat. No. 3,650,714, to Farkas, discloses a method of coating diamond particles with a thin titanium or zirconium layer, typically up to 5% by volume of the diamond, by metal vapor phase deposition. The coating's inner surface forms a carbide with the diamond. A second layer of a less oxidizable metal, such as nickel or copper, can then be applied to protect the inner layer from oxidation. Diamond particles coated by titanium are commercially available from DeBeers and General Electric.
Tensile testing of double layer coated diamond having an inner layer such as titanium or chromium and an outer layer such as nickel shows that fracturing occurs at the interface between the inner and outer metal layers. This suggests that nickel does not alloy or otherwise bond well with the underlying carbide and that double layer coated grits according to Farkas may not significantly improve overall grit retention. Bonding can also be weakened by oxidation of the inner titanium or chromium layers during the nickel coating process.
In U.S. Pat. No. 4,339,167, Pipkin discloses metal coating diamond or CBN particles with titanium, manganese, chromium, vanadium, tungsten, molybdenum or niobium by metal vapor deposition. It has been found, however, that the carbide formers chosen by Pipkin, do not bond strongly enough to the diamond crystals to improve their grit retention for many high stress applications, or are susceptible to oxidation. As discussed above, the outer metal layers used to protect inner layers from oxidation, do not adequately bond to the inner layer.
U.S. Pat. No. 4,378,975, to Tomlinson, describes a first, thin metal coating up to 10% by volume of the particle and a second, wear resistant coating of between one and two times the radius of the particle. The inner coating is preferably chromium and the outer coating is a nickel-iron based alloy, a metal bonded carbide or boride, or silicon carbide. A metal bonded carbide is typically a mixture of a metal or alloy and a carbide. Although not elaborated upon, a metal bonded carbide can be bonded to the first metal layer by metallurgical bonds between the metal of the metal bonded carbide and the first layer. There is essentially no direct chemical bonding between the carbide itself and the first layer.
In U.S. Pat. No. 3,929,432, Caveney controls the duration of heat treatment to improve the bond between an inner titanium layer and diamond in a double coated diamond particle having an outer layer of nickel, or other alloying metals. The patent does not address the problem of weak bonding between the titanium and the outer alloying metal coating.
Metal coatings have also been used in resin matrices to insulate the superabrasive particle, decreasing thermal degradation, and to improve the bonding of grit to the matrix. Nickel coated grit has improved retention in resin matrices over uncoated grit due to improved adhesion with the matrix. The bond between the grit and the nickel, however, is still weak and is a cause of grit loss.